LTC3631_15 Datasheet, PDF (13/22 Pages) Linear Technology – High Efficiency, High Voltage 100mA Synchronous Step-Down Converter

LTC3631
Applications Information
Output Voltage Programming
For the adjustable version, the output voltage is set by
an external resistive divider according to the following
equation:
V_OUT
=
0.8V
•

¹⁺

R1^
R2


The resistive divider allows the V_FB pin to sense a fraction
of the output voltage as shown in Figure 6. Output voltage
can range from 0.8V to V_IN.
V_OUT
V_FB
LTC3631
GND
R1
R2
3631 F06
Figure 6. Setting the Output Voltage
To minimize the no-load supply current, resistor values in
the megohm range should be used; however, large resistor
values should be used with caution. The feedback divider
is the only load current when in shutdown. If PCB leak-
age current to the output node or switch node exceeds
the load current, the output voltage will be pulled up. In
normal operation, this is generally a minor concern since
the load current is much greater than the leakage. The
increase in supply current due to the feedback resistors
can be calculated from:
ΔI_VIN
=
 _{RV1O+URT 2}



•



V_OUT
V_IN



Run Pin with Programmable Hysteresis
The LTC3631 has a low power shutdown mode controlled
by the RUN pin. Pulling the RUN pin below 0.7V puts the
LTC3631 into a low quiescent current shutdown mode
(I_Q ~ 3µA). When the RUN pin is greater than 1.2V, the
controller is enabled. Figure 7 shows examples of con-
figurations for driving the RUN pin from logic.
V_SUPPLY
LTC3631
RUN
V_IN
4.7M LTC3631
RUN
3631 F07
Figure 7. RUN Pin Interface to Logic
The RUN pin can alternatively be configured as a precise
undervoltage lockout (UVLO) on the V_IN supply with a
resistive divider from V_IN to ground. The RUN pin com-
parator nominally provides 10% hysteresis when used in
this method; however, additional hysteresis may be added
with the use of the HYST pin. The HYST pin is an open-
drain output that is pulled to ground whenever the RUN
comparator is not tripped. A simple resistive divider can
be used as shown in Figure 8 to meet specific V_IN voltage
requirements.
V_IN
R1
R2
R3
RUN
LTC3631
HYST
3631 F08
Figure 8. Adjustable Undervoltage Lockout
Specific values for these UVLO thresholds can be computed
from the following equations:
Rising
V_IN
UVLO
Threshold
=
1.21V

• _1+

R1^
R2


Falling
V_IN
UVLO
Threshold
=
1.10V

• _1+

R1 ^
R2
+
R3


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